fix package config path when running precompiled vm tests from source (#1737)

Now that we don't support `.packages` files, if you pass one it won't fall back on the package_config.json any more, which we were apparently relying on.

I did retain support for `.packages` files for now as well, which we could drop eventually but I would want to wait until we can bump our min SDK to a stable version that also doesn't support it.
4 files changed